Method and apparatus for drying pulp including heat exchange and temperature equalizing zones

ABSTRACT

PULP FLAKES ARE DRIED BY ENTRAINING THEM IN A FLOWIN GAS AND CONDUCTING THE GAS-FLAKES MIXTURE THROUGH A MULTIPLICITY OF DRYING UNITS, EACH OF WHICH INCLUDES A HEAT-EXCHANGER SECTION FOR TRANSFERRING HEAT INTO THE GAS-FLAKES MIXTURE AND A TEMPERATURE-EQUALIZING SECTION WHERE THE TEMPERATURES OF THE GAS AND FLAKES ARE BROUGHT INTO SUBSTANTIAL EQUALITY. THE CONCENTRATION OF PULP FLAKES IN THE GAS IS MAINTAINED AT A LEVEL SUCH THAT THERE IS NO MORE THAN ABOUT 0.35 KG. DRY WEIGHT OF PULP FOR EACH 1.0 KG. OF GAS. THE SUPPLY OF HEAT IN THE HEAT-TRANSFER ZONE AND THE MOISTURE CONTENT OF THE FLAKES AND GAS PROCESSED THROUGH THE EQUIPMENT ARE CONTROLLED SO THAT THE GAS DISCHARGED FROM THE DEVICE HAS A WET-BULB TEMPERATURE NOT LESS THAN ABOUT 60*C. AND A DRY-BULB TEMPERATURE OF NOT GREATER THAN ABOUT 120*C. THE HEATED PORTION OF THE FLOW PATH ALONG WHICH THE GASFLAKES MIXTURE FLOWS HAS A HYDRAULIC DIAMETER NOT GREATER THAN ABOUT 200 MM.

DING HEAT NES v 4 Sheets-Sheet 1 P" B.- o. A. HEDSTROM ETHQD AND APPARATUS FOR DRYING PULP INCLU EXCHANGE AND TEMPERATURE EQUALIZING Z0 FiledFeb. 15. 1972 SUPPLY OF PULP FLAKES PREHEATER AIR SUPPLY CONTROL April 30, 1974 a. o. A. HEDSTROM 3,303,093

I METHOD AND APPARATUS FOR DRYING PULP INCLUDING HEAT v EXCHANGE AND TEMPERATURE EQUALIZING ZONES 4 Sheets-Sheet 2 Filed Feb. 15, 1972.

CONTROL 27\ STEAM SUPPLY CONDENSATE RECEPTACLE 3 9 8 0 Ru n 3m GS mm D 0 UZ L CG N N II PU wfi OP mm .513 D m ED w RH v ow 1mm mm mm P P E G m N H m D\ m L 4 l 7 w 9 H 1 0 3 A M 4 Sheets-Sheet 5 Filed Feb. 15, 1972 April 30, 1974 a. o. A. HEDSTROM 9 METHOD AND APPARATUS FOR DRYING PULP INCLUDING HEAT EXCHANGE AND TEMPERATURE EQUALIZING ZONES Filed Feb. 15, 1972 4 Sheets-Sheet 4 fig'l 1H 111 United States Patent Int. or. Firm 3/10, 17/10 U.S. Cl. 162-100 Claims 8 Claims ABSTRACT OF THE DISCLOSURE- Pulp flakes are dried by entraining them in a flowing gas and conducting the gas-flakes mixture through a multiplicity of drying units, each of which includes a heat-exchanger section for transferring heat into the gas-flakes mixture and a temperature-equalizing section where the temperatures of the gas and flakes are brought into substantial equality. The concentration of pulp flakes in the gas is maintained at a level such that there is no more than'about 0.35 kg. dry weight of pulp for each 1.0 kg. of gas. The supply of heat in the heat-transfer zone and the moisture content of the flakes and gas processed through the equipment are controlled so that the gas discharged from the device has a wet-bulb temperature not less than about 60 C. and a dry-bulb temperature of not greater than about 120 C. The heated portion of the flow path along which the gasflakes mixture flows has a hydraulic diameter not greater than about 200 mm.

BACKGROUND OF THE INVENTION ous disadvantages. Generally, the efliciency of utilization of heat is 10w, inasmuch as large quantities of hot gas are required to conduct heat into the material to be dried. The inefficient utilization of heat is an especially great disadvantage in pulp mills requiring large amounts of hot water, which is usually the case in mills producing kraft pulp. In many flash-drying processes, there is a risk of the attainment of excessively high gas temperatures in drying that may very well cause deterioration in the quality of the pulp. The gas conveying systems in many known devices are subject to wide fluctuations in flow characteristics that result in frequent obstructions and blockages of the equipment and require the equipment to be shut down and the blockages cleared, to the serious detriment of eflicient production.

SUMMARY OF THE INVENTION 1 There are provided, in accordance with the present invention, a method and an apparatus for flake drying pulp that overcome the disadvantages of presently known methods and equipment and offer significant advantages in their own right. In particular, the method and apparatus of theinvention overcome many of the problems associated with the heat transfer conditions between a heatedg'as and pulp flakes by providing heat exchange in several separate zones along the path through which the pulp gas mixture is conveyed. The ratio of pulp flakes to each having a heat-transfer zone and a temperature gas and the properties of the conveying system are established and controlled to provide stable slow conditions that promote high efficiency of heat utilization and a minimum of risk of blockages in the equipment. The invention enables the pulp to be dried under suitable temperature conditions while producing a hot effluent gas that can be utilized efliciently in heating water for use in other parts of the mill.

In accordance with the invention, the pulp flakes are entrained in a gas, air being quite appropriate, in an amount not greater than 0.35 kg. dry weight of pulp per 1.0 kg. of gas. Preferably the air or other gasis preheated prior to introducing the pulp flakes. The gasflakes mixture is then conducted through a conduit system that is constructed to provide a multiplicity of units,

equalizing zone, arranged in series. In particular, each unit comprises a heat-transfer zone constituted by a length of conduit containing a heat exchanger, preferably in the form of a multiplicity of spaced-apart tubes extending parallel to the axis of the conduit and of ap-.

propriate length and surface area to provide a desired heat input in each zone. Preferably, the tubes are heated by conducting steam at a low saturation pressure in the range of from about 2 atmospheres to 12 atmospheres. As the mixture of pulp and flakes flows past the heat-exchanger section of each unit, heat is transferred into the gas and flakes. Each unit of the apparatus further includes a temperature-equalizing zone downstream from the heattransfer zone. As the pulp flows through the temperatureequalizing zone of each unit, the temperatures of the flakes and the gas become substantially equalized, thus ensuring high efliciency and preventing the attainment of overly high gas temperatures. Consequently, the production of a high quality pulp is ensured.

Stable flow conditions in the equipment are maintained by controlling a combination of factors. As already mentioned, the ratio of pulp flakes to gas is held below 0.35 kg. of pulp, dry weight, per 1.0 kg. of gas. It has been found that a pulp loading significantly above that limit increases the likelihood of formation of fiber networks or agglomeratcs. In addition, thecross-section of the conduit system is dimensioned and shaped so that the velocity profile at any zone in the system does not have any unstable parts of the same order of size as the flocs oraggregations of pulp that tend to form in the system due to the inherent property of fibrous pulp flakes to form networks or flocs. It has been found that the'fiocs or aggregations of pulp flakes generally have a size on the order of 50 mm. to mm., a range which corresponds roughly to the unstable part of a velocity profile in a conduit having a hydraulic diameter of about 200 mm. Accordingly, an important feature of the invention is the construction of the flow system's'uch that the hydraulic diameter at any heated zone in the system does not exceed about 200 mm.

In order for the heat in the eflluent gas leaving the equipment to be utilized economically for heating water for general use in the mill, the ratio of pulp to gas is controlled so that, taking into account the moisture content of the pulp, the rate of introduction of moisture into the system is suflicient to produce an eflluent gas from Patented Apr. 30, 1974.

general the rem'ainingsectio'ns constitute at leastpor tions of the temperature-equalizing zones. Adjacent pairs of heat-transfer sections and temperature-equalizing sections are connected together by elbows to place' the temperatureequalizing section downstream'bf the corresponding heattransfer ""s'ecti'o n 'in eachunit. The cross-sectional" area of each elbow decreases in proportion to 'the dis tance' of any given cross-section from the upstream end to ensure against" the establishment of a stagnation zone at "the downstream, radially inwardjend of the elbow and thus prevent turbulence and back flow and a possibility of an unstable flow condition and possible blockage of the conduit.:-Preferably, the'flow of the gas-air'mixture is augmented periodically by conducting the mixture through. conveyor fans. Such fans may be located near the downstream-"end-of each temperature-equalizing zone of the system. The conveyor fan may provide the connection between eachadjacent pair of units in the system. Advantageously, the conduit sections are oriented vertically with the elbows uppermost and the conveyor fans lowermost. In that case, the fans are conveniently and readily mounted on footmgs, which is'ofcon'siderable advantage from the point of view of an efiicient'structural arrangement.

The initial cost of the equipment is significantly less than previously known equipment for flash-drying of pulp, primarily because heatingtakes place at. several stages with temperature equalization between the'heattransfer stages. Consequently, the quantities of gas tobe handled will be small, and the conveyor fans and other components of the equipment may be commensurately small and thus of lower cost. A unit having conveyor sections arranged vertically in parallel, side-by-side relation occupies a minimum of space and is structurally durable. The equipment can be located outside with appropriate coverings required only for the motors and instrumentation. This means further SaVlIlgS in cost.

The method and apparatus can readily be'used with a gas other than air, for example a chemically inert gas, such as air having its oxygen content removed' The l quid. removed duringthe drying process can be extracted readily, for example, by the acetone extractionproce'ss.

DESCRIPTION.OF THEDRAWINGS, 1

For a better understanding of the invention, reference maybe made to-the following description of an exemplary embodiment, taken in conjunction with the .figures of the accompanying drawings, in which: 'f

FIG. 1 is a side-'elevational view of two units of the embodimennthe' view being in' generally schematic and diagrammatic form and showing acen'ter portion of the equipment broken out toreduce "the height'of the'figure;

'FIG. 2 is an end view inelevationgof the apparatus of FIG. 1, the 'view' being taken genei-ally along a broken plane, as represented by e lines 1-1 in FIG. remain the direction of the arrows; H

FIG. 3 is a side view in' section of the upper portion of a heat-exchanger section of a unit'of FIGS. 1 and 2', the view being on asomewhat'lar'ger scale thanFIGS. .1 .2 w FIG."4--is a side view in section'of thelower portion a DE ON F EXEMPL YE QP1MNT,.. Referring to FIGS. 1 and-2 of the drawings, reference numeral 1 designates centripetal fans that are driven by motors 3 and have generally conical inlet pipes 5 connected to the suction sides of the fans. The output side of each of the fans is connected by a tapere'd'outletj duct section 9 to the lower end of a vertical conveyor duct section 7. The conveyor duct 7 is generally square in cross-section (see FIG. 5) and contains a' head-exchanger unit composed of a multiplicity of spaced-apart pipes 11 that extend parallel to the axis ofthe duct 7. Connected to the top of each duct 7 is a .180 ?v elbow duct 13 that connects the upper end of the duct 7 to the upper end of a tapered duct section 15. Each duct section 15 is connected at its lower end to aductsecti'onj17, which is also oriented vertically, is ofsubstantially square cross-section and is laterally adjacent the section 7. The

. lower portion of the duct section 17 is constructed to transform the cross-section of the duct system from square to substantially circular, and the'circular lower end of the section 17 is connected to the :conical inlet'5 ofan adjacent fan 1. v

As will be described in more detail below, and as illus-' trated in FIGS. 6 and 7 of the drawings, the duct sections 9, 7, 13; 15, 17-and.5 constitute one unit of a multiple unit system composed of several of the basic units mounted adjacent each other and connected-to each other in series. The major portion of eachductsection 7 constitutes a heat-transfer zone of eachunit, and the duct sections 13, 15, 17 and 9 constitute a temperature-equalizing zone of each unit. 1

The elbow 13 is constructed with a varying cross-sectional area that provides a smooth reduction in area from its upstream end to its downstream end so that the outlet area is about 30% to 40% less than the inlet area. The reduction occurs at the inner periphery of the bend and eliminates a stagnation zone that would exist in an elbow of uniform cross-sectional area, thereby eliminating eddy currents and turbulence that would disrupt flow'and 11 ssibly cause blockage of the duct.

FIG. 1.,of the drawings illustrates schematically .a supply system for delivering a mixtureof gas, such asap, and pulp flakes into the first unit of the-apparatusAir is supplied from the atmosphere through a control valve thatcanbe set to regulate the amount of air, is conducted through a vpr eheater and thence through ,appropnate piping into the inlet 5 of the fan lot; the first un t. Pulp flakes delivered from the pulp de-watering and pressing of wheat-exchanger section of a unit, also on a larger scale than FIGS. 1 and 2;

FIG-,5 is a'top view in cross-section takenthrough' a heat-exchanger section of one'of the units of the embodi ment of FIGS. 1 and '2; i

- FIG. 6 is a side-elevation alview ofa complete installaof the arrows in FIG. 6.

equipment, appropriately shredded into flakes,.are fed, such a's by a rotary feeder, into the inlet pipe. The rate of feedf of the pulp flakes is controlled either by controlling the output of the press and shredding device ,or by controlling the rotary feeder, the latter formof control being illustrated in the drawing.

,FIGQZ illustrates schematicallythe supply of steam from a source through an inlet pipe 19, a control valve 21 and branch lines 23 leading to the upper ends of the heat exchanger pipes 11. The steam condenses in the pipes, and the condensate is drawn ofl through outlet manifolds 25, a valve 27 and an outlet. 29, to an appropriate condensate receptacle or other processingequip ment. I

As shown in FIG. 3, the inlet manifolds 23 to the heat exchanger pipes 11 have V-shaped deflectors 31 mounted on their under sides to divert the [flow and maintain smooth flow conditions. As shown in FIG. 4, the lower "ends of the steam pipes 11 are tapered, and the condensate'manifold outlets are of relatively small diameter. Therefore, relatively smooth flow conditionsexist at the lower .ends-of'the heat-exchanger section.- It will also be observed from FIGS. 3 and 4"that the tops and bottoms of the pipe sets are staggered vertically, again to, eiihance the flow conditions in the heat exchanger sections.

Referring to FIGS. 6 and 7, an exemplary embodiment of a complete installation of the apparatus of the invention consists of nine units of the form described above and illustrated in some detail FIGS. l-to 5. The nine units are arranged in atriangular-configuration with three units along each side" of'the trianglel'The gas-pulp flake mixture is conducted to the first unit through an inlet 33, passes in series through the units along the leg of the system shown at the left of FIG. 7, is conducted to the top three units, beginning with the left-most unit, and finally is conducted through the series of three units along the right leg of the triangle in FIG.- 7. The dry flakes and exhaust gases are discharged through an outlet conduit 35. The apparatus illustrated in FIGS. 6 and 7 is provided with an appropriate cover structure, but as already mentioned, it would be appropriate to provide covers only for the fans and instrumentation of the device, the

conduits being left exposed.

It will be evident to those skilled in the art that various additional controls will be provided, and this is well within the ordinary engineering skill. For example, the fan motors may be controlled to vary the speed of the fans and therefore the flow-rates invarious units of the equipment. Temperature and humidity detecting devices will be employed throughout to assist in controlling the equipment.

,In operation, air is introduced at a controlled rate through the preheater into the inlet of the first fan 1 (see FIGS. 1 and- 7). Pulp flakes are supplied at an appropriate rate to the incoming air flow. The ratio of pulp flakes to incoming air should not exceed 0.35 kg. of pulp flakes per 1.0 kg. of air, a ratio that has been found to reduce significantly a risk of excessive fioccing or agglomeration due to the formation of fiber networks and loss of fiber mobility. The ratio of pulp to air is also controlled to introduce sufficient moisture into the system to ensure that the gas exhausted from the device is of relatively high humidity, as mentioned above.

The pulp-flakes mixture is propelled by the fan 1 of the first unit upwardly through the heat-exchanger section of that unit where heat is transferred from the pipes into the mixture of air and pulp flakes. As already mentioned, the steam condenses in the pipes and is withdrawn through the condensate collector pipes 25. The predominate heat transfer is into the air rather than into the flakes and, accordingly, the mixture leaves the heat exchanger with the air at a significantly higher temperature than the flakes. The mixture passes around the elbow 13 and downwardly through an open vertical conduit that constitutes the major portion of a heat-equalization zone in each unit. During the passage of the mixture through the temperature-equalizing section, heat exchange takes place between the air and the pulp flakes, thereby to reduce substantially the temperature difierential. Equalization of the temperature of the gas and the temperature of the air is assisted by a velocity increase at the downstream end of the elbow, a velocity decrease as the cross-section expands through the tapered conduit section 15 and another velocity increase through the conical inlet 5 to the fan and at the outlet from the fan.

The above process is repeated in each unit of the equipment, the pulp-flakes mixture passing from the delivery fan into the heat-exchanger section and through the temperature-equalizing section of each unit and passing through the several units in series. At each unit, additional heat is added and additional moisture removed from the flakes. Between each input of heat in the heatexchanger units, temperature equalization occurs, thus precluding the establishment of excessively high air temperatures in the equipment. As the material leaves the last unit through the outlet 35, the air has a wet-bulb temperature of at least 60 C. and a dry-bulb temperature not in excess of about 120 C. The mixture, after discharge through the outlet pipe, may be conducted through a cyclone or cyclone system, or other appropriate separation devices, for removing the dried pulp flakes from the moist air. The air may be conducted to water heating equipment and represents a source of useful heat for general purposes, particularly the heating of water for general mill use.

It is evident that the processing conditions in the equipment may vary significantly, depending upon the char acteristics of the pulp and the results desired. An example of a process applied to a particular product follows.

For the production of ADT pulp supplied in the form of flakes de-watered and pressed into a web and then shredded into small flakes having a dryness of 48%, a nine-stage unit of the form illustrated in FIGS. 6 and 7 of the drawings may be used. Each heat exchanger has a height of 15 meters and provides a total heat exchange area of 3000 sq. meters for the total of nine stages, the heat exchanger area being equally divided among the nine stages. The temperature-equalizing sections of each unit have a height of 18 meters. In addition, the lower portion of each heat exchanger section provides an additional temperature-equalizing zone having a height of approximately three meters. The steam tubes in each heat exchanger are arranged with a center-to-center spacing of 90 mm., and have external diameters of 50 mm.

Air flow conditions are maintained in the equipment to provide a velocity of about 25 meters per second in the square cross-section ducts. Steam is delivered to the system at 3 kp./cm. Ambient air is drawn in at 20 C. and is preheated in the preheater unit to about 110 C. The incoming air cools upon contact with the incoming pulp flakes through a temperature of approximately 60 C. The first heat exchanger raises the temperature of the air to about C. and the air and flakes equalize in the temperature-equalizing section of the first unit to about 50 C. The heat exchanger of the second unit raises the air temperature to about C., and the temperature is equalized at about 58 C. In the third stage, the temperature of the air is raised to about 96 C. and the pulp and air temperatures equalize at about 64 C. Similar increments of heat input and equalization occur in the remaining stages so that the final stage provides a heating of the air to about 120 C. and a temperature equalization of between the temperature of the pulp and the air at about 100 C. The pulp leaves the equipment with a dryness of about 90%.

The above-described embodiment and example of the invention are intended to be merely exemplary, and many variations and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included in the scope of the invention, as defined in the appended claims.

What is claimed is:

1. A method of drying pulp flakes comprising the following steps: (I) entraining the pulp flakes in a gas in an amount not greater than about 0.35 kg. dry weight of pulp to 1.0 kg. of gas, (2) establishing and maintaining a flow of the gas-flakes mixture along a confined path having a hydraulic diameter not exceeding about 200 mm. at any heated region thereof, (3) transferring heat into the gasflakes mixture in a heat transfer zone along the path, (4) substantially equalizing the temperatures of the gas and the flakes in a temperature-equalizing zone of the path downstream from the heat-transfer zone, (5) repeating steps (3) and (4) in successive series of heat-transfer and temperature-equalizing zones, and (6) controlling the supply of heat in the heat-transfer zones and the moisture content of the flakes and gas so that the gas discharged from the final temperature-equalizing zone has a wetbulb temperature not less than about 60 C. and a drybulb temperature of not greater than about C.

2. A method according to claim 1 and further comprising the step (7) of preheating the gas prior to step (1).

3. A method according to claim 1 and further comprising the step (8) of imparting kinetic energy to the gasflakes mixture at least at one zone along the confined path.

4. A method according to claim 3 wherein kinetic energy is imparted to the gas-flakes mixture following each step (4) and prior to each step (3).

5. Apparatus for drying pulp flakes comprising a conduit system defining a confined path for a flow of pulp flakes entrained in a gas, means for establishing a flow of pulp flakes entrained in a gas through the conduit system, a multiplicity of heat exchangers in the conduit system for transferring heat into the gas-flakes mixture in heatexchanger zones, and at least one blower in the conduit system for imparting kinetic energy to the gas-flakes mixture, the conduit of the conduit system having a hydraulic diameter not exceeding about 200 mm. in any heatexchanger zone along its length and including a plurality of laterally adjacent, generally parallel conduit sections, alternate ones of said conduit sections containing spacedapart heat exchangers and the remaining intervening conduit sections defining at least portions of temperatureequalizing zones located between successive heat-exchanger zones and downstream from each heat exchanger for substantial equalization of the temperatures of the flakes and the gas prior to entry into the next heat-exchanger zone, pairs of conduit sections being interconnected by elbows at one end and constituting units each containing one of the heat-exchanger zones and one of the temperature-equalizing zones, the downstream end of each of said units being connected in series by one of said at least one blower with the upstream end of a succeeding adjacent unit, each heat exchanger including a plurality of tubes carrying a heating medium, the tubes being located within and oriented generally parallel to one of said alternate conduit sections.

6. Apparatus according to claim 5 and further comprising means for controlling the supply of pulp flakes and gas to the conduit to maintain an amount of flakes not greater than 0.35 kg. dry weight for each 1.0 kg. of gas.

7. Apparatus according to claim 6 and further comprising means for controlling the supply of heat to the heat exchangers and the moisture content of the gas and flakes processed in the apparatus to provide in the gas leaving the last temperature-equalizing zone a wet-bulb temperature of not less than C. and a dry-bulb temperature of not greater than C.

8. Apparatus according to claim 5 wherein the crosssectional areas of each elbow diminish smoothly in relation to the distance in the downstream direction from the upstream end.

References Cited UNITED STATES PATENTS 1,766,843 6/1930 Soderlund et al 34-10 3,256,614 6/1966 Dunbar 3457 R 1,755,373 4/1930 Soderlund et al 34-10 3,264,751 8/1966 McEntee, Jr. 263-21 A 3,148,955 9/1964 Nichols 34-50 3,627,630 12/1971 Gagnon 162100 3,309,785 3/1967 King 26321 A 3,458,942 8/ 1969 Martin et al 3457 A ROBERT L. LINDSAY, J 11., Primary Examiner R. V. FISHER, Assistant Examiner US. Cl. X.R. 

